The increasing level of integration within an electrical integrated circuit (IC) leads to both higher data rates and larger number of IC interconnections. The inherent signal speed has recently increased to 3 GHz with indications that it will reach 10 GHz and beyond in the future. The number of pin connections is also increasing. A single IC now requires close to 2000 connections with indications that this number will increase towards 5000-plus.
In conventional PCBs, an electric signal dissipates while propagating through a signal line. The signal dispersion is proportional to the signal frequency. The higher the signal frequency the lower the bandwidth of the signal line used for connecting one chip to another on the board. The tangent loss of the dielectrics are high, the bandwidth of the interconnects gets limited to the point that the high speed signal cannot be sent over a longer distance compared to dielectrics with lower tangent loss.
The interconnect technology associated with this increasing level of integration cannot keep up. With the continued growth in integration density of complementary metal-oxide semiconductor (CMOS) technology and the clock frequency of chips (e.g. microprocessors), the aggregate bandwidth required between future generation chips and chipsets is increasing sharply. Microwave circuits require substrate materials that have tight control of dielectric constants as well as low loss tangents. Today's printed circuit boards (PCBs) are made of uniform epoxy-glass composite material (Trade name: FR4). The FR4 technology is well known to be reliable and cost effective. FR4 has material characteristics, however, which limit usage in high speed applications when using conventional interconnection structure. This is because FR4 has high dielectric loss which limits the bandwidth of the interconnects. As signal rates and pin counts continue to rise, the disparity between off-chip and on-chip signal speeds will also continue to rise.
High frequency materials for use in off-chip interconnection technology are known. Alumina, Polytetrafluoroethylene (PTEF), Duroid, Rogers 4003, and GTEK all bridge the gap by providing low loss tangents and tight control on dielectric constants, but with a high manufacturing cost associated. The cost of implementing the production of new material in the PCB fabrication process is estimated to be more than ten times the conventional methods. The new materials are also incompatible with conventional FR4 processing, further increasing the cost.
In sum, the conventional PCB technology used for off-chip interconnection is not feasible with increasing signal speeds. Existing conventional electrical interconnects can only achieve a certain level of bandwidth. Beyond this level, the development of a completely new manufacturing technology is needed. Current solutions have proven too costly for the PCB industry.